There has been significant interest in developing high power, high repetition rate, solid state, pulsed laser amplifiers. In these systems, a small master laser oscillator is used to generate low power pulses. Preferably, the pulses have a single longitudinal and transverse mode. The pulses are directed into an amplifier cell which includes a solid state gain medium such as a Nd:YAG rod. The solid state gain medium is typically excited by a flashlamp but recently diode lasers have been used as the excitation source. The pulses from the master laser are amplified by the energy stored in the gain medium.
One problem which has limited the development of higher power amplifiers is that the excitation of the gain medium generates a significant amount of heat which increases the temperature of the rod. The laser rod is typically cooled from the outside leading to radial temperature gradients within the rod. As the pump power is increased, nonuniform temperature distributions can create thermally induced stresses which distort the wavefront of the beam and degrade the performance of the laser.
Various approaches have been proposed to remove or compensate for these wavefront distortions. One of the more promising approaches is described in U.S. Pat. No. 4,734,911, issued Mar. 29, 1988, to Bruesselbach and also described in a related article "Phase Conjugation: Reversing Laser Aberrations," Photonics Spectra, page 95, August 1986. In this approach, the properties of a phase conjugate mirror are used to reverse the wavefront of a beam so that when it is sent back into the gain medium, the distortions can be compensated. (See also, "Performance of a Nd:YAG Oscillator/Amplifier with Phase-Conjugation via Stimulated Brillouin Scattering," Carr and Hanna, Applied Physics B, 36,83-92 (1985)).
FIG. 1 illustrates a simplified schematic layout of a prior art solid state amplifier 10 using a phase conjugate mirror of the type described in the above cited references. As illustrated in FIG. 1, low power pulses are generated by a small master oscillator 12. The output pulses are coupled into the amplifier gain medium 14. The gain medium is excited by a flashlamp 16. As noted above, the thermal loading of the gain medium will create phase front distortions in the beam.
If the distorted wave were reflected back into the gain medium using a conventional reflector, the phase front distortions would be further increased by the thermally induced stresses in the crystal. In contrast, if the wavefront is reflected by a phase conjugate mirror 18 (PCM), the distortions are reversed. By passing the reversed distortions back through the gain medium, the distortions can compensated. A quarter-wave plate 20 can be located between the mirror 18 and the medium 14 to rotate the polarization of the beam so that it may be coupled out of the amplifier using a polarizing splitter 22.
There are various types of phase conjugate mirrors which can be used in the layout illustrated in FIG. 1. The above cited Bruesselbach patent suggests using a stimulated scattering type medium such as stimulated Brillouin scattering (SBS) or stimulated Raman scattering (SRS). A sealed transparent tube filled with Freon defines a common SBS element.
The approach described by Bruesselbach appears to have been satisfactory for moderate repetition rates and therefore moderate thermal loading of the amplified gain medium. More specifically, Bruesselbach indicates that his system was operational up to about 10 Hz. It would be desirable to operate a system at a repetition rate of 100 Hz or higher. When the repetition rate is increased to this level, the amount of pump power which is coupled into the gain medium is significantly greater. In this situation, the thermally induced stresses create significant birefringence which induces strong depolarization effects in the beam. In fact, depolarization ratios of greater than fifty percent can be expected with flashlamp powers in the kilowatt regime. The depolarization ratio is defined as the energy in the unwanted (orthogonal) polarization divided by the total energy in the beam.
Unfortunately, the ability of the phase conjugate mirror to reverse the phase front of a beam (defined as the fidelity of the phase conjugate mirror) is dependent on the polarization purity of the incoming beam. Thus, if the beam is depolarized to any great extent, a true phase front reversal will not be achieved by a simple phase conjugate mirror based on an SBS cell. If the distorted phase front returning into the gain medium is not the same as that emerging from it, then the distortions will not be fully compensated resulting in degraded performance of the system.
Accordingly, it is an object of the subject invention to provide an approach for minimizing the depolarizing effects created by thermally induced stress birefringence.
It is another object of the subject invention to create a system where the depolarization effects are minimized so that the phase front distortions can be compensated.
It is a further object of the subject invention to provide an amplifier system where the depolarizing effects of the gain medium are minimized.
It is still another object of the subject invention to provide an solid state laser amplifier which can operate at a repetition rate of greater than 100 Hz without degraded performance.
It is still a further object of the subject invention to provide a solid state laser amplifier which can efficiently operated over a full range of repetition rates and thermal loading without significant adjustment.